OFDM (Orthogonal Frequency Division Multiplexing) generally refers to a frequency division multiplexing technique, in which a serial data stream is transmitted parallely (and simultaneously) over multiple orthogonal carriers. OFDMA (Orthogonal Frequency Division Multiple Access) refers to a multi-user multiple-access communication technique that uses OFDM (Orthogonal Frequency Division Multiplexing).
OFDMA systems may contain multiple base stations (BS), with each base station designed to link users (mobile or subscriber stations) in corresponding geographical areas called cells, and may thus constitute a multi-cellular system. Each cell may, at any given point in time, contain multiple users (subscribers/subscriber stations), that may be either fixed (not moving) or mobile.
In OFDMA systems, one or more sub-carriers are grouped to form a sub-channel, and one or more sub-channels may be assigned to a subscriber. To use available frequency resources (bandwidth) efficiently, the same frequency (same subcarrier) may be allocated to multiple users in the same time interval (or overlapping time intervals) provided the users are located in different cells. This technique is called frequency reuse. As an example, a user located within a cell (‘cell-A’) may be allocated a set of sub-carriers having frequencies f1, f2, and f3, while another user in a different cell ('cell-B′, which may be a neighboring/adjacent cell with respect to ‘cell-A’) may be allocated sub-carriers f3, f4 and f5, with the same frequency (subcarrier f3) being allocated to both the users.
In general, one or more sub-carriers may be allocated in the same time interval to multiple users, to enable efficient frequency (bandwidth) utilization. However, such an approach may cause interference. In the above example of reuse of frequency f3, proximity of the two users with respect to each other may cause interference (often termed co-channel interference or CCI) in the band corresponding to frequency f3, and thereby degrade communication.
Interference diversity, with respect to OFDMA, is generally a measure/degree of non-commonality of sub-carriers allocated to subscribers. In general, a greater degree of interference diversity corresponds to fewer sub-carriers being common to any two subscribers in an OFDMA system. Thus, an allocation scenario in which a maximum of only one sub-carrier is common among any two subscribers in the OFDMA system is deemed to provide higher interference diversity compared to an allocation scenario in which a maximum of five sub-carriers are common among any two subscribers. It may be appreciated that greater interference diversity among subscribers generally translates to lesser information loss due to interference.